1. Field of the Invention
The present invention relates to a filer for ultrafiltration.
2. Prior Art
For particles, whose average dimensions are less than 2 microns, there are essentially three methods readily available to efficiently separate or filter particles by size. Processing times are usually measured in hours and sample volumes to be separated are typically a very small fraction of a milliliter. They generally involve the use of a supporting medium, where particles are easily trapped and recovery of all starting materials is often difficult if not impossible.
Filtration of particles on the order of tens or hundreds of Angstroms by size is frequently referred to as Molecular Filtration or Ultrafiltration. The filters with the highest selectivity or sharpest cutoff in particle size, are composed of thin semipermeable membranes made from a variety of polymer materials. These membranes are anisotropic or asymmetric in cross section and may not be used at temperatures above 35.degree. C. A model membrane molecular filtration system consisting of uniform pores, and a solid spherical solute 20% as large as the pore will theoretically pass only 85% of the solute, the remainder being retained within the membrane. The resulting filtrate will be only partially fractionated. Efficient particle size separation between two narrow size distributions using membrane molecular filtration often requires size differentials of over an order of magnitude.
The best filtration rate of membranes at 50 PSI is typically about 0.1 ml/min/cm.sup.2 for particles on the order of 100's of Angstroms in diameter. However, membranes are themselves pressure sensitive and consideration has to be given to the advantages of increased flux as the pressure is increased versus membrane compression which skews the effective filtration diameter of the filter.
Control pore filter glasses are made with nominal pore sizes as small as 7.5 nanometers. The pores are randomly oriented within the glass particles and have typical pore size distributions of approximately 10%. The glass particles are typically 50-500 microns in size and are themselves randomly oriented when packed into filter columns. The pores are therefore randomly oriented across the pressure gradient of the filter column, broadening the dispersion for identical materials transport and lowering the resolution of this filtering system. In addition, material may flow around the voids surrounding the individual glass particles composing the column, further reducing the filtering resolution. To make up for these deficiencies, the length of the filtering column must usually be considerably extended.
Gel permeation chromatography can be used as a very sensitive method to separate particles of different size for certain ranges of particle size distributions, around room temperature. The gel acts as a supporting medium for the transport of materials and generally retains the majority of the smaller particles. If the separated particles are to be collected, the medium must eventually be removed from the material of interest. This can be a very difficult and laborious procedure.